The instrument to be described is both a special and a general test instrument. When used as a special test instrument in conjunction with a vehicle sensing system, as mentioned above, the instrument provides accurate measurements as to the overall inductive characteristics of the loops associated therewith, and of the response of the loops to vehicles passing within their confines, while the loops are operating under their actual working conditions.
The instrument of the invention has particular utility for use in conjunction with digital vehicle detection systems as described in copending applications Ser. Nos. 364,492, now U.S. Pat. No. 3,875,555, and 443,299, now U.S. Pat. No. 3,873,964, both of which were filed in the name of the present inventor.
Copending application Ser. No. 443,299, for example, discloses a vehicle detection system which responds to changes in the inductance of a wire loop embedded in a roadway to sense the presence of a vehicle. The loop and associated lead-in connections are used in the system as a frequency-determining network for a Class C oscillator, and a digital circuitry is used to measure the frequency of the oscillator.
The test instrument to be described, likewise, has a Class C oscillator in its circuit, and it is equipped with a receptacle which may receive the plug of a suitable connector which may be easily connected into the loop lead-in network of such a vehicle detection system, without the need to make or break any connections in the system. The test instrument of the invention is basically a frequency counter with a range, for example, of 1 megahertz. The embodiment to be described has a fixed, one second time base. The indicator of the instrument consists of six numeric seven-segment/light emitting diode displays which together provide a one hertz resolution for the instrument.
When the test instrument of the invention is connected in circuit with the loop/lead-in network of a vehicle detection system, its Class C oscillator generates an approximate sine wave near the resonant frequency of the loop/lead-in network, and changes in the resonant frequency of the network due to vehicle influence or environmental changes can be monitored. The one-second time base of the test instrument is obtained in the embodiment to be described, by counting down the output of a 100 kilohertz oscillator, as will be described in more detail subsequently herein.